Nonvolatile programmable digital potentiometers, known also as nonvolatile reprogrammable electronic potentiometers or digitally controlled potentiometers, have been, for about the last 15 years, some of the most emerging front-line integrated circuits (ICs). They are used in LCD (Liquid Crystal Display) screen adjustment, volume control, automated product calibration, remote adjustment of equipment, signal processing, and other applications requiring the replacement of mechanical potentiometers.
FIG. 1 illustrates an embodiment of a nonvolatile reprogrammable potentiometer 10, as it is described in U.S. Pat. No. 4,668,932, the disclosure of which is hereby incorporated by reference. The potentiometer 10 includes a series chain of impedance elements 40 and additional circuitry for accessing the chain. The chain 40 has a first element 13, a last element 15, and one or more intermediate elements 12. The first element 13 is connected to a first terminal 14 (called high impedance terminal or H in other references), and the last element 15 is connected to a second terminal 16 (called low impedance terminal or L in other references). A plurality 44 of electrically controllable switches 20 connects every internal node (or tap) of the chain 40, to a third terminal 18 (called wiper terminal or W in other references). An output line 36 from a selecting block 38 controls each switch 20; the block 38 determines which electrically controllable switch 20 is closed at a given time. The selecting block 38 consists of a counter 24 connected to a decoder 22. The state of the counter may be incremented or decremented by signals on counter input terminals 28 and 30. The terminal 28 is used to apply count increment signal INCR, and the terminal 30 to apply an up/down control select signal UP/DN SEL. Storage block 60 stores the status of the counter 24 in response to a preselected signal. The storage block 60 consists of a nonvolatile memory 25, a control circuitry 26, and a circuitry 27 used to detect the status of the counter 24. The chip select line CHIP SEL 33 and the control line 32 are used to control when the memory 25 is updated and when it updates the counter 24, the control line 32 being connected to the supply voltage of the circuit.
Prior-art digital potentiometers, such as the potentiometer 10 of FIG. 1, tend to produce errors when operated as voltage dividers having a low impedance load connected to the third or wiper terminal 18.